As you already learned, PHP does not get entered in the editor. To show a link only to admins create a custom shortcode to display the link through this hook: https://formidableforms.com/knowledgebase/frm_display_entry_content/   Your code needs to test whether the current user is an admin or not and then branch accordingly. 

You can also create a custom shortcode that takes parameters and passes back the URL you want to display (or nothing, in the case where it's a non-admin).